KNCalculating Member Forces in Truss Structures

[MMCS]

Attached is a picture of the truss. I am trying to calculate the member forces.

I know a moment is = force * perpedicular distance, but in this case, for example, if i am taking moments about A, Would the equation be

-(1*2KN)+(1*c) = 0

or using an alternative perpedicular distance

-(2.4*2KN)+(0.8*c) = 0

For my preliminary working, assuming i have used correct perp distance i have

ƩF_x = 0
ƩF_y = 0
ƩMoments about a point = 0

A_y + B_y - 2KN = 0

Momments about A = -(2KN * 2.4) + 1*ReactionC = 0
-4.8KN = reactionC



Sub into first eq

A_y +(-4.8) -2 = 0
A_y = 6.8

To find out the reaction in member AB

I have

6.8 - T_ab Cos(tan^-10.8/1) = 0
T_ab = 6.8/Cos(tan^-10.8/1) = 8.738

I have the answer to be 2.56

 

[SteamKing]

In taking moments, the 2kN force has the same direction as the vertical distance on 1m, so its moment will be 0. If you get confused, you can always calculate M = r x F, where x is the cross product operator.

 

[MMCS]

In taking moments, the 2kN force has the same direction as the vertical distance on 1m, so its moment will be 0. If you get confused, you can always calculate M = r x F, where x is the cross product operator.

Im don't quite understand sorry, i would understand if the force was acting through the point i am taking momoments about but how can the 2KN force x distance away not cause a moment

 

[SteamKing]

The moment at A due to the force at E is 2 kN * 2.4 m = 4.8 kNm, which is why your initial moment equation is incorrect.

See the definition of moment:
http://www.splung.com/content/sid/2/page/moments

 

[MMCS]

Using the value of of 4.8 but the same working i end up with a final equation of

2.8/cos(38.66) = 3.59

 

[MMCS]

Ok i realize where my confusion was on this problem thanks for your help